Many methods have been practically applied and proprosed for reducing the pollutants in the exhaust gas. In these methods, the exhaust gas treatment have been improved in the equipment or its operation of the exhaust gas sources and treatment of the exhaust gas. Thus, the emission of pollutants in the exhaust gas have been considerably suppressed, but it is still unsatisfactory and the new excellent techniques have been requested for further reducing the pollutants in the exhaust gas.
The substances occuring in the combustion of fuels, for example, nitrogen oxides, carbon-containing particles, carbon monoxide, hydrocarbons are regarded as pollutants in the exhaust gas. Nitrogen oxides and carbon-containing particles are cited as particularly troublesome substance in the exhaust gas from the internal combustion engine.
The method for removing the nitrogen oxides from the exhaust gas with a supported solid catalyst, so called “three way catalyst” has been practically applied, and can be applied only to very narrow range of air-fuel ratio, so called “window”, but cannot be applied to the exhaust gas from Diesel engine or lean-burn gasoline engine in an excessive oxygen atmosphere, as the theoretical air-fuel ratio is required to be precisely controlled.
Recently, a NO x storage-reduction three way catalyst system to be applied to the lean burn gasoline engine has been practically applied (WO 93/07363, S.A.E. paper 950809, Catalysis Today 27 (1996) 63). But this system has a problem on the performance deterioration caused by the sulfur in the fuel.
An method to use molten salts for treating an exhaust gas including nitrogen oxides has been disclosed. (J.P. publication No 34564/1970. J.P. publication No 5633/1976. U.S. Pat. No. 3,671,185, U.S. Pat. No. 3,718,733, U.S. Pat. No. 3,754,074, U.S. Pat. No. 3,817,715). It relates to the absorption and removal of nitrogen oxides with a molten alkali carbonate at an elevated temperature higher than 350° C. Consequently, it may be difficult to secure a constant performance in the treatment of the exhaust gas from such a vehicle engine of which the operation conditions are drastically fluctuated. Further, the removal of the nitrogen oxides by absorption is poor in the presence of carbon dioxide. Further more, in particular, a problem remains in the initial cleaning of the exhaust gas from the start of the operation until the temperature of the exhaust gas treatment apparatus reaches the high actuation level.
The carbon-containing particles in the exhaust gas can be effectively removed by capturing them with a filter which is often blocked with the carbon-containing particles and required to be frequently exchanged with a fresh one. The oxidative removal of the carbon-containing particles accmulated on the filter has been tried in the presence of a catalyst supported on the filter, the filter is often damaged by the steep temperature rise.
In a proposal, the carbon-containing particles in the exhaust gas are used to remove the nitrogen oxides and both of them are expected to remove simultaneously. But, a solid catalyst is used in the process and the solid carbon-containing particles can not be satisfactory captured and reacted.